Nonwoven products consisting of cellulose or cellulose/synthetic fibers that possess wet strength characteristics typically are produced by impregnating the fibers with an emulsion polymer which is capable of crosslinking. Crosslinking results in an polymer that imparts water resistance to the nonwoven products. Many self crosslinking polymers are formed by copolymerizing a variety of monomers such as vinyl acetate, ethylene, vinyl chloride and alkyl esters of acrylic acid with a functional monomer such as N-methylolacrylamide (NMA) that provides the useful capability of self crosslinking to itself and to cellulosic surfaces to form self-sustaining webs.
The following references are cited as representative of the prior art:
Air Products Technical Bulletin “AIRFLEX® 105 Emulsion for Nonwovens” discloses aqueous based vinyl acetate-ethylene-NMA polymer emulsions and their use in producing nonwoven products. To facilitate cure through the NMA group, a variety of dicarboxylic acids, such as, oxalic acid, citric acid, and inorganic acid salts such as sodium bisulfate and ammonium chloride are employed.
Air Products Technical Bulletin 151-9317, “AIRFELX® 108 Ultra -Low Formaldehyde Emulsions for Nonwovens” discloses vinyl acetate-ethylene emulsions for use in producing nonwoven products. The resins are self crosslinking and provide good wet strength performance. The addition of an acid catalyst is recommended to accelerate and promote cure of the binder during the heating phase.
Air Products Technical Bulletin entitled “AIRFLEX® Ethylene-Vinyl Chloride (EVCL) Emulsions for Nonwovens and Textiles” discloses the use of ethylene-vinyl chloride polymer emulsion as binders for nonwoven applications. Enhancement of the wet tensile properties is achieved by effecting crosslinking with an external crosslinker such as melamine or urea formaldehyde resins.
U.S. Pat. No. 3,380,851 discloses nonwoven fabrics bonded with an interpolymer of vinyl acetate, ethylene and from about 0.5 to 10% of an intrinsic acrylamide based upon vinyl acetate. NMA is the self crosslinking monomer employed. The binder is applied to the fibers in an amount from 20 to 100% on a dry weight basis. Then, addition of a mineral acid catalyst is suggested as a mechanism for promoting curing of the interpolymer.
U.S. Pat. No. 3,787,232 discloses the formation of vinyl and vinylidene halide polymers prepared by the emulsion polymerization of vinylidene halide monomer with a carboxylic acid monomer and an N-alkylol amide monomer for use in coating fibers. The low pH latex polymers are alleged as being curable at low temperature
U.S. Pat. No. 4,449,978 discloses a nonwoven product bonded with an emulsion binder comprising vinyl acetate, ethylene, and a 50/50 mole mixture of NMA/acrylamide that provides nonwoven products having low residual free formaldehyde content and good tensile properties. Cure of the binder to achieve crosslinking of the NMA is accomplished by addition of an acid catalyst.
U.S. Pat. No. 4,590,102 discloses the production of nonwoven products by impregnation with an emulsion binder based on vinyl acetate, ethylene, and NMA or an emulsion based on a vinyl chloride, ethylene, and NMA interpolymer. A low temperature curing agent having a pKa ranging from about 1 to 2 is employed to facilitate cure. Examples of low temperature curing agents included organic dicarboxylic acids such as oxalic acid and maleic acid.
U.S. Pat. No. 5,244,695 discloses a coating composition for producing filters comprising from 10 to 90% of a fully hydrolyzed polyvinyl alcohol, and from 0 to 90%, preferably from 30 to 70% of an aqueous polymeric emulsion consisting of an aqueous emulsion of a vinyl acetate-NMA polymer or an ethylene-vinyl chloride polymer. Both aqueous polymer emulsions may be used in combination in an amount from 15:85 to 85:15. The saturated nonwoven finds utility in air, oil, fuel and vacuum filter media. An acid catalyst is preferably added to the formulation to aid in the crosslinking of the resin.
U.S. Pat. No. 3,752,733 and U.S. Pat. No. 3,758,429 disclose ethylene-vinyl chloride interpolymers suited for forming nonwoven products. The '733 patent discloses the incorporation of from 0.1 to 10% acrylamide and the '429 patent discloses the incorporation of NMA to produce a crosslinkable interpolymer. To enhance the laundering and dry cleaning resistance of the nonwoven product the interpolymers are condensed and cured by reacting with an aminoplast.